The present invention relates to electric transformers, especially distribution transformers of the overhead type, and to protective equipment therefor.
Investigations have been made in recent years of failures of common designs of single-phase overhead (pole-type) distribution transformers caused by lightning current surges. As a result of these investigations, it has been found that anomalous failures of such transformers are attributable to lightning surges entering the transformer via the normally unprotected low voltage terminals, causing failure of the high voltage coil. By "anomalous" failures is meant failures that occur despite state-of-the-art lightning protection. Thus, it was found that high voltage arrester protection of the high voltage coil by conventional arrester equipment was ineffective in avoiding this problem. Particularly affected were shell-type transformers with noninterlaced mid-grounded divided low voltage winding, i.e., of 120/240 secondary voltage rating. It appears that the damaging voltages were induced by oppositely poled current surges on each half, or section, of the divided low voltage winding.
While shell-type or core-type transformers with interlaced low voltage coils have been significantly less susceptible to surge currents of the same magnitude, it appears that the adoption in recent years of non-interlaced construction by many manufacturers of the shell-type units may account for increased premature or anomalous failures of distribution transformers.
Investigations relative to the above discussed problem are reported in detail in IEEE paper 82MW093-3 by C. W. Schoendube et al, presented in New York City in February 1982. This paper, in pages 1-9, discusses the effect of the above described surge currents on various types of transformers, and is incorporated herein by reference.
It is well-known to provide transformers with surge arresters externally of the tanks thereof to shunt the currents associated with lightning strikes to ground and thus protect the transformer elements against extreme voltage peaks by such transient surges. Aside from the fact that the usual arrester arrangements have not adequately protected distribution transformers of the above-noted non-interlaced shell type against the anomalous failures discussed, the known arrester arrangements, especially of the external type, may require relatively long leads, have a variety of mounting hardware, include porcelain or similar types of enclosures which on occasion are subject to explosion or other causes of breakage, are relatively expensive, or have other difficulties associated therewith.